Substances are forms of matter that have a specific composition and distinct properties. They can be classified into two main categories: elements and compounds. Elements are pure substances made of only one type of atom, while compounds consist of two or more different atoms chemically bonded together. Substances can exist in different states, including solid, liquid, and gas, depending on temperature and pressure. Each state has unique characteristics, such as shape and volume. Understanding substances is essential in fields like chemistry, where scientists study their interactions and transformations, contributing to advancements in technology and medicine.